1. Field of the Invention
The invention relates to a high-frequency amplifier of a radio transmitter, comprising an amplifier stage the gain of which can be automatically controlled by a gain control feedback coupled from a subsequent amplifier stage or the output of the high-frequency amplifier.
2. Description of the Prior Art
An automatic power control is used in many radio transmitters to maintain the transmission power constant irrespective of the different factors affecting it. Moreover, it must be possible especially in mobile telephone systems to select the transmission power of a mobile phone depending on the current radio link, for instance according to the information transmitted from the base station of a mobile phone.
Previously, a series transistor which regulates the current passing through an amplifier transistor in a manner depending on the actual transmission power has been used in one of the amplifier stages of a transmitter for controlling the transmission power. The problem with a transmission power control of such kind is that a series transistor consumes an excessive amount of current, which reduces the efficiency of the transmitter and shortens the charging interval of battery-driven transmitters.
Another problem with known high-frequency amplifiers is that the power control may affect the frequency stability of the transmitter by changing the load impedance of a modulator or a voltage-controlled oscillator (VCO) coupled before the amplifier stages, and thereby the frequency. To remove this problem there have been previously been attempts to increase the separation between the amplifier stages and the VCO by means of attenuators coupled between them.